The present invention relates to a torque split control system for 4WD vehicles and method, and more particularly, to a forward and aft torque distribution control system for variably controlling torque distribution to be split to front and rear wheels of a four-wheel drive vehicle by controlling coupling torque of an electronically controlled clutch and a method of controlling forward and aft torque distribution for the four-wheel drive vehicle.
Considerable research and development work has been undertaken in the related art to suitably control forward and aft torque distribution of a four-wheel drive (hereinafter referred to as 4WD) vehicle. An attractive approach to the suitable control of the torque distribution uses a manual mode changeover switch adapted to produce mode selection signals, a controller responsive to the mode selection signals, and an electronically controlled clutch controlled by the controller so as to control torque distribution at a variable torque split ratio to be split to front and rear wheels of the vehicle. The mode changeover switch produces a 2WD (two-wheel drive) mode signal (a clutch uncoupling mode signal), a LOCK mode signal (a clutch torque locking signal) and an AUTO mode signal (an automatic clutch torque regulating signal), which are manually selected. Such a 4WD vehicle is for example disclosed in Japanese Patent Application Laid-Open Publication No. S63-170129.
Such an electronically controlled clutch is constructed of a clutch of the type which is actuated by hydraulic pressure that is electronically controlled and which has multiple clutch plates adapted to be coupled with hydraulic pressure. With the use of working oil as working medium to produce coupling torque, the 4WD vehicle is not subjected to undesired noisy sounds that would otherwise occur in a four-wheel (4W) drive unit owing to clutch coupling and twisting motions of a drive shaft and propeller shaft during running of the vehicle in forward or rearward directions under a circumstance in that the vehicle is fully steered rightward or leftward.
However, in the electronic controlled clutch of the type wherein the multiple clutch plates are directly urged without the use of working medium which produces hydraulic pressure, coupling torque is obtained with the clutch through a mechanical actuating cam mechanism. With such an actuating cam mechanism, the 4W drive unit comes into a situation where torque through the electronically controlled clutch is not smoothly distributed to wheels, that is, the 4W drive unit becomes internally replete with the torque in the electronically controlled clutch (hereinafter simply referred as replete) during running mode of the vehicle in the forward or rearward directions under the condition in that the vehicle is fully steered rightward or leftward. This leads to noisy sounds produced in the 4W drive unit.
That is, when the LOCK mode is selected and engine drive torque is split over to the front and rear wheels on the basis of torque distribution ratio 50:50, steering the vehicle at a slow speed and at a full extent during running in the forward direction (or in the rearward direction) at the LOCK mode essentially increases the magnitude of torque to be replete in the 4W drive unit owing to twisting motions of the drive shaft and the propeller shaft. Under such a condition, if the vehicle is shifted in the rearward direction (or in the forward direction), the twisting motion tends to be rapidly returned with a resultant noisy sound.
In particular, when the LOCK mode is selected, the actuating cam mechanism of the electronically controlled clutch operates as follows. That is, as shown by the arrows in FIG. 3, when the LOCK mode is selected, a control cam is retracted rearward of the vehicle at the LOCK mode and a main cam is pushed forward of the vehicle at the LOCK mode, with balls losing binding power between separated opposing cam recesses such that the balls are located in an outermost area in a radial direction due to centrifugal force. Subsequently, when the twisting motion is rapidly returned, the balls, which remains in an unstable state without binding power, is apt to strike against the opposing cam recesses which tend to close, with a resultant knocking sound to produce the undesired noisy sound in the 4W drive unit.
One technology for addressing these issues is to uncouple the electronically controlled clutch by considering the halt condition of the vehicle to release coupling torque thereof until the vehicle comes to a halt state during the forward running state and is shifted to the rearward running state for thereby preventing the 4W drive unit from being replete with torque.
However, the 4W drive unit still suffers from the noisy sound unless coupling torque is released until the vehicle is shifted from the halt condition in the forward running state (or in the rearward running state) to the rearward running state (or the rearward running state).
The present invention has been made to address these issues and has an object of the present invention to provide a forward and aft torque distribution system for a 4WD vehicle which ensures a performance of a controller at a LOCK mode while avoiding a noisy sound from being produced in a drive unit when the LOCK mode is selected.
It is another object of the present invention to provide a method of controlling forward and aft torque distribution in a 4WD vehicle so as to overcome various problems encountered in the related art practices.
According to one aspect of the present invention, there is provided a forward and aft torque distribution control system for a four-wheel drive vehicle. The system comprises an electronically controlled clutch to control forward and aft torque distribution to be transferred to front and rear wheels, a mode changeover switch selectively producing mode selection signals involving at least a LOCK mode selection signal and an AUTO mode selection signal, a vehicle speed sensor detecting a vehicle speed and producing a vehicle speed signal, and a controller responsive to the mode selection signals and the vehicle speed signal and controlling an electronically controlled clutch to control forward and aft torque distribution to be split to front and rear wheels. The controller has control modes involving at least a LOCK mode to allow the electronically controlled clutch to be locked in a coupled state, and an AUTO mode to allow the electronically controlled clutch to be automatically controlled so as to alter forward and aft torque distribution in dependence on running conditions of a four-wheel drive vehicle. When the vehicle speed becomes less than a vehicle""s halt discriminating vehicle speed indicative of a vehicle""s halt state during the LOCK mode, the controller allows the electronically controlled clutch to be released. Please note that halt xe2x80x9cdiscriminativexe2x80x9d vehicle speed is used interchangeably herein with halt xe2x80x9cdiscriminatingxe2x80x9d vehicle speed.
In other words, a forward and aft torque distribution control system for a four-wheel drive vehicle comprises an electronically controlled clutch controlling forward and aft torque distribution to be transferred to front and rear wheels, mode changeover means for selectively producing at least a LOCK mode selection signal and an AUTO mode selection signal, vehicle speed sensing means for detecting a vehicle speed and producing a vehicle speed signal, and 4WD control means for controlling an electronically controlled clutch to control forward and aft torque distribution to be split to front and rear wheels, responsive to the mode selection signals and the vehicle speed signal. The 4WD control means has control modes involving at least a LOCK mode to allow the electronically controlled clutch to be locked in a coupled state, and an AUTO mode to allow the electronically controlled clutch to be automatically controlled so as to alter forward and aft torque distribution in dependence on running conditions of a four-wheel drive vehicle. When the vehicle speed becomes less than a vehicle""s halt discriminating vehicle speed indicative of a vehicle""s halt state during the LOCK mode, the 4WD control means allows the electronically controlled clutch to be released.
Besides, a method of controlling forward and aft torque distribution control for a four-wheel drive vehicle of one aspect of the present invention prepares an electronically controlled clutch to control drive torque to be transferred to front and rear wheels, prepares a controller having control modes involving at least a LOCK mode to allow an electronically controlled clutch to be locked in a coupled state and an AUTO mode to allow the electronically controlled clutch to be automatically controlled so as to alter forward and aft torque distribution in dependence on running conditions of a four-wheel drive vehicle, selectively produces mode selection signals involving at least a LOCK mode selection signal and an AUTO mode selection signal, detects a vehicle speed and producing a vehicle speed signal, and controls the electronically controlled clutch with the controller to control forward and aft torque distribution to be split to front and rear wheels in response to the mode selection signals and the vehicle speed signal such that when the vehicle speed becomes less than a vehicle""s halt discriminating vehicle speed indicative of a vehicle""s halt state during the LOCK mode, the controller allows the electronically controlled clutch to be released.